The present invention relates to wavelength monitoring circuit for monitoring the output of a laser light source and to an optical communication system that uses this wavelength monitoring circuit, and more particularly to a wavelength monitoring circuit for monitoring variation in the wavelength and variation in the output of laser light or for maintaining the wavelength and output level at desired values, and also to an optical communication system.
The rapid popularization in recent years of information communication, of which the Internet is representative, has resulted in the abrupt increase in network traffic, and this increase has raised the demand for trunk networks and access networks having greater capacity. This demand has been met by employing optical fiber communication technology through WDM (wavelength division multiplex) in which a plurality of signals each having a different wavelength are multiplexed on an optical fiber. With the aim of achieving still higher capacity of communication volume, various techniques are being developed for broadening the pass wavelength bandwidth, increasing the number of channel wavelengths, and narrowing the channel wavelength interval. Research and development has been particularly active in wavelength division multiplexing having a high degree of multiplexing, i.e., DWDM (Dense Wavelength Division Multiplexing), for such uses as the trunk line of a network. Development of this type of technology calls for stabilization of the wavelength of the laser light that is used as the signal light.
FIG. 1 shows the typical composition of an arrayed waveguide diffraction grating that is used for obtaining laser light of a plurality of wavelengths separately from a laser light source of particular wavelengths. Arrayed waveguide diffraction grating 11 is made up by: a single or a plurality of input waveguides 12 formed on a substrate (not shown in the figure); a plurality of output waveguides 13; channel waveguide array 14 having differing curvatures in the same direction; input-side slab waveguide 15 for connecting input waveguides 12 to channel waveguide array 14; and output-side slab waveguide 16 for connecting channel waveguide array 14 to output waveguides 13.
The paths of multiplexed signal light that is entered from input waveguides 12 are spread by input-side slab waveguide 15, following which each is directed into channel waveguide array 14 at equal phase. The intensity of entered light is not equal at each incident point of input slab waveguide 15, the intensity increasing towards the central portion of input-side slab waveguide 15 in a substantially Gaussian distribution.
A predetermined difference in the length of the optical path is provided between successive arrayed waveguides that make up channel waveguide array 14: the length of the optical paths are set to successively increase or decrease, so that light beams guided by each of the arrayed waveguides arrive at output slab waveguide 16 with a predetermined phase difference. In actuality, the occurrence of chromatic dispersion causes the equiphasal surface to tilt depending on a wavelength, and the light beams therefore form images (converge) at differing positions depending on a wavelength on the interface of output-side slab waveguide 16 and output waveguide 13. Output waveguide 13 is arranged at positions in accordance with wavelength, and differing wavelength components can therefore be derived from output waveguide 13.
The central wavelength of this type of arrayed waveguide diffraction grating 11 is extremely sensitive to changes in the refractive index of the waveguide material caused by changes in temperature. The wavelength that is derived from output waveguide 13 must therefore be monitored to control temperature so as to prevent variations in wavelength.
We now refer to FIG. 2 in which is shown an example of an output monitor/control device proposed in the prior art. This device is disclosed in Japanese Patent Laid-open No. 31859/99.
In the figure, laser light 21 emitted from laser diode 20 passes through cut-off filter 22 and is incident on beam splitter 23. Light beam 24 that has been transmitted by beam splitter 23 is applied to a measurement device or communication device (not shown in the figure) and used as light having a stabilized wavelength. Light beam 25 reflected by beam splitter 23 is directed to optical bandpass filter 26, this optical bandpass filter 26 being arranged perpendicular to light beam 25. As a result, a part of reflected light beam 25, transmitted by filter 26, impinges on first photo-diode 281 (the transmitted light is denoted as reference numeral 27 in the figure) . The remaining light, reflected from filter 26 passes through beam splitter 23 and impinges on second photodiode 282 (the reflected light is denoted as reference numeral 29).
The outputs 311 and 312 of first and second photodiodes 281 and 282 are applied to output-ratio calculation means 32 for calculating the ratio of the outputs 311 and 312. The thus-obtained monitor signal 33 is then applied to wavelength control means 34. Wavelength control means 34 controls the wavelength of emitted light of laser diode 20 to maintain this output ratio at a prescribed value. The wavelength control of the light radiated from laser diode 20 is realized by varying the drive current of laser diode 20 or by altering the ambient temperature.
FIG. 3 shows the wavelength characteristics of each part of the device shown in FIG. 2, (a) showing the wavelength characteristic of cut-off filter 22, (b) showing the transmittance of optical bandpass filter 26, and (c) showing the reflectance of optical bandpass filter 26. Since cut-off filter 22 is a low-cut filter the cut-off wavelength of which is prescribed to be slightly shorter than the central wavelength λ1 of band-pass filter 26, the level of the light received by first photodiode 281 varies as shown in FIG. 3(d) The received light level of second photodiode 282 varies as shown in FIG. 3(e). In the figures, λ2 denotes a half-value wavelength and λ3 denotes a zero-attenuation wavelength on the high-wavelength side of the filter 26. The emitted light wavelengths of laser diode 20 can be controlled by setting the ratio of the outputs of these photodiodes 281 and 282 to a prescribed value.
FIG. 4 shows another device similar to the device shown in FIG. 2. Parts in FIG. 4 identical to those shown in FIG. 2 are identified by the same reference numerals and redundant explanation is omitted. This device is disclosed in Japanese Patent Laid-open No. 022259/2000.
Laser light 21 radiated from laser diode 20 is directed into optical interference filter 41, which is tilted with respect to the optical axis. Transmitted light 42 impinges on first photodiode 281 and reflected light 43 impinges on second photodiode 282.
Outputs 441 and 442 of first and second photodiodes 281 and 282 are supplied to output ratio calculation means 32 and the output ratio is calculated. Thus obtained monitor signal 45 is applied to wavelength control means 34. Control of the emitted wavelength of laser diode 20 is implemented such that the output ratio is maintained at a prescribed value.
We now refer to FIG. 5, in which is shown the principal components of a device that employs an etalon for monitoring and controlling wavelength. This device is disclosed in Japanese Patent Laid-open No. 223761/2000.
Laser light 21 radiated from laser diode 20 is branched by first beam sampler 51 to make two branched beams 52 and 53 having a small angle with each other. These branched beams 52 and 53 are directed into etalon 54 and received at photodiodes 551 and 552. The outputs of photodiodes 551 and 552 have a wavelength-current characteristic in which the phases are shifted with respect to each other. Calculation unit 56 normalizes the wavelength-current characteristic and uses the portion of the wavelength-current curve that has good linearity to calculate a wavelength deviation from the reference wavelength.
The transmitted light 57 through first beam sampler 51 is incident on second beam sampler 58 to similarly obtain two branched beams 61 and 62 having a small angle with each other. One of the branched beams 61 is received as is by third photodiode 553. The other branched beam 62 is received by fourth photodiode 554 after passing through slope filter 64. Calculation unit 56 finds the reference wavelength of the incident beam from the wavelength-transmittance characteristic of slope filter 64. Calculation unit 56 obtains an accurate wavelength of irradiated laser light 21 by adding this reference wavelength and the wavelength deviation.
Of these examples of output monitor/control devices of the prior art, the devices shown in FIG. 2 and FIG. 4 control the wavelength of laser light by adjusting the ratio of the outputs 311 and 312, or 441 and 442 of two photodiodes 281 and 282 to a prescribed value. As a result, there is the problem that the control of laser light wavelength is restricted only within a narrow wavelength range in which the characteristics curves of outputs complementarily change according to the slopes in opposite directions, as shown in (d) and (e) of FIG. 3. Taking the example shown in FIG. 2 and FIG. 3 to further explain this point, the wavelength range in which control is possible when using cut-off filter 22 as shown in (a) of FIG. 3 is limited to wavelengths that are longer than λ1. This is because inclusion of shorter wavelengths results in the existence of two wavelengths that take the same ratio (λ2 and a wavelength that is symmetrical to λ2 with respect to λ1), and control to a specific wavelength is therefore not possible.
While wavelengths are restricted to longer wavelengths than λ1 in the device shown in FIG. 2, control of wavelengths in the range longer than wavelength λ3 is disabled because the transmittance of optical bandpass filter 26 substantially reaches a minimum as the wavelength approaches wavelength λ3 at which reflectance falls off to substantially zero. Of course, the wavelength range between the two wavelengths λ1 and λ3 can be somewhat extended by selecting the characteristics of optical bandpass filter 26. When this wavelength range is extended, however, the slopes of the transmittance and reflectance curves of optical bandpass filter 26 become relatively gentle and the change in the outputs 311 and 312 of the two photodiodes 281 and 282 , respectively, versus change in wavelength becomes smaller, thereby resulting in deterioration in the accuracy for controlling wavelength to a desired value.
In order to solve the above-described problem regarding the restricted wavelength range of control, means such as the arrayed waveguide diffraction grating shown in FIG. 1 can be employed to individually control the wavelengths of optical signals over a wide range of wavelengths. Such a solution, however, entails the use of a number of output monitor/control devices each having a characteristic constitution for a wavelength band, and this approach has the problems of the considerable time required for development as well as the difficulty of reducing the cost of the device.
On the other hand, a wavelength monitor/control device that employs an etalon such as shown in FIG. 5 does not suffer from this problem and is capable of accurate processing over a relatively wide range of wavelengths. The device shown in FIG. 5, however, employs two beam samplers 51 and 58 and entails an increase in the number of other parts that accompanies the use of these components. As a consequence, such a device has the problems of increased bulk as well as the difficulty in reducing the cost of the device.
It is an object of the present invention to provide an output monitor/control device which by itself can be applied with high accuracy to a relatively wide range of wavelengths, allowing a simplified device architecture.
It is another object of the present invention to provide an output monitor/control device that can handle a large number of wavelengths with basically the same constitution.
It is further object of the present invention to provide an optical communication system employing such an output monitor/control device.